MUTATION TESTING




Can we write perfect tests? - Maybe!

MY TESTING JOURNEY


First I HATED it.
Then I FEARED it.
Later I did not do ENOUGH.
Finally to MUCH.

TEST METRICS




How can you prove the tests are solid?

And how do you make sure you got all the edges?

TEST TO CODE RATIO


I'm joking yeah.




Lets move on.

LINE COVERAGE

a start

 def cover_me(input)   input ? :foo : :bar end

100% covering test case :(

 expect(cover_me(true).to be(:foo)

Misses to specify the else branch.

BRANCH/Statement Coverage

(more sophisticated)

def cover_me   side_effect_a # No test for this one :(   side_effect_bend

100% covering test case :(

 expect { cover_me }.to change { side_effect_b }.from(initial).to(other)
Misses to specify side effect a.

MUTATION COVERAGE




My unscientific claim:

Mutation-Coverage > Statement-Coverage > Line-Coverage > Test-To-Code-Ratio

TESTING RUBY



It is even harder!



Sub 100% coverage guarantees you deploy code with bugs.

REAL STORY

def fooend
def bar baz ? fooo : other # note misspelled method call "fooo"end

Only running this code can identify the spelling error!
(Some IDEs will still try but fail)

DEFINITION



A mutation testing tool changes (mutates) your code strategically and expects your tests to FAIL!

Mutants with failing tests are KILLED.
Mutants without failing tests are ALIVE.

Fear ALIVE mutants. They should be dead!

What to MUTATE


CODE!

HOW?


String#gsub is NOT an option.


Transformable representations of code must be used.

AST-BASED


Some code:
def foo  barend
Some AST:
(whitequark/parser)
(def :foo  (args)  (send nil :bar) 

BYTECODE BASED

some bytecode (rbx)
============= :__script__ ==============
0000:  push_rubinius              
0001:  push_literal               :foo
0003:  push_literal               #<:compiledcode foo="" file="x.rb">
0005:  push_scope                 
0006:  push_variables             
0007:  send_stack                 :method_visibility, 0
0010:  send_stack                 :add_defn_method, 4
0013:  pop                        
0014:  push_true                  
0015:  ret
================= :foo =================
0000:  push_self                  
0001:  send_method                :bar
0003:  ret                        

Not used by any ruby tool so far.

MUTATION EXAMPLE

Original:
def cover_me(input)  input ? :foo : :barend 
Mutation:
def cover_me(input)   true ? :foo : :barend
The test?
 expect(cover_me).to eql(:foo) 
Still passes! - Mutant is ALIVE.

KILLING - Mutation

Mutation
def cover_me(input)   true ? :foo : :barend

Killing Test!
expect(cover_me(true)).to eql(:foo)expect(cover_me(false)).to eql(:bar)

He is dead Jim!

THE BAD EXAMPLE

Nobody is perfect.

def square_root_bug(value)  3end
Test
 expect(squire_root_bug(9)).to be(3)


No way of covering this invalid implementation via mutation testing.

Mutation Operators



  • literal / primitive and compound
  • statement deletion
  • conditional
  • binary connective replacment
  • argument deletion / rename / swap
  • unary operator exchange
  • bitwise
  • many, many more!

In the REAL WORLD

Subjects:  424       # Amount of subjects(methods) being mutated
Mutations: 6760      # Amount of mutations mutant generated ~13 / method
Kills:     6664      # Amount of successfully killed mutations
Runtime:   5123.13s  # Total runtime
Killtime:  5092.63s  # Time spend killing mutations (~83min)
Overhead:  0.60%
Coverage:  98.58%    # Coverage score Alive:     96


These numbers are outdated.
mutant-0.3.0.rc1 is 35-50% faster.

REPORTING


evil:ROM::Mapper::Dumper#identity:/home/mbj/devel/rom-mapper/lib/rom/mapper/dumper.rb:18:08a61
@@ -1,6 +1,6 @@
 def identity(object)
   header.keys.map do |key|
-    object.send(key.name)
+    object.public_send(key.name)
   end
 end 

SPEED

Mutation testing is slow.


For N mutants your tests get to run N times.



Write real unit tests, to make your test execution fast.

TEST SELECTION




Selecting the correct subset of your tests is the key.

Known-Strategies: 

  • Brute force
  • (Method)-Name based mapping
  • Use Line-Coverage metrics to identify subject - test mapping.

ISOLATION




Mutations must be isolated from each other.

Strategies: fork(), sandboxing

EQUIVALENT MUTANTS

Original:
i = 0while i != 10  do_something  i+=1end
Equivalent mutant:
i = 0while i < 10  do_something  i+=1end 
No observable change, reported as alive mutation.

INFINITE Runtime

Original
while expression  do_somethingend
Mutation
while true
  do_something
end 

Only killable via time, or refactoring.

Hunting CHEAT SHEET


Refactor mutations away, avoid literals
 array[0] => array.first 

Only use syntactic constructs when needed
 ::Foo => Foo

Do not pass literal defaults into methods
 string.to_i(10) => string.to_i
To be continued...

FINALLY

Thank you for listening!

https://github.com/mbj/mutant

Contact:

Markus Schirp

https://github.com/mbj
https://twitter.com/_m_b_j

THANKS


%w(  j-j-k dkubb whitequark  solnic snusnu postmodern    txus and_all_i_forgot).shuffle

For various achievements and all the related work. 

mutation-testing

By Markus Schirp

mutation-testing

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